In the US they do not re-check ID's at the gates.
Your boarding pass is scanned by a barcode reader, and the computer does seem to pick up invalid passes, but I don't think that is the point.
The TSA has made some kind of a big deal about only allowing "ticketed passengers" into the gate areas. However, their check of this comes down to ensuring you have a piece of ID that matches the name on a piece of paper you bring from home.
To use an example from the story, you could have 20 people who are not travelers each smuggle a component of a bob through security (a portion of a liquid, ptex, etc.) and then give those components to a single flyer with a valid ticket (who would have presumably gone through security with NO contraband at all, so as not to burn his identity if he accidentally went through a line with an alert agent).
There is also the simple matter of basic vulnerability testing. If you have to spend $500 on a ticket to get a trip through the TSA line, it's very costly to test the edges of the system. If you can go through the line 4x per day at 3 different airport terminals, or multiple airports that are in close proximity to each other, then you can easily run 100 test scenarios in a week about how the lines are managed, processed, etc.
As an example, I travel frequently and don't like dealing with the full-body scanners. In most airports at the busy times they "randomly" select some passengers to just go through the metal detectors because the body-radiators are slow. With about 80% accuracy I can watch how the lines are being handled and time my fiddling around with items on the xray belt to be "randomly" selected to skip the full body scanner. It takes a few cycles of observation to start to see the patterns though.